To increase useful power from an electronic power source, heat must be dissipated efficiently and effectively from the power supply components. The overall junction-to-ambient thermal resistance (θja) decreases after proper improvements are made. Traditionally, to lower this thermal resistance the power components of a board mounted power (BMP) device are soldered onto an insulated metal substrate (IMS) circuit board. End users can then mount a heat sink or cold plate onto the opposite side of the IMS board for better cooling effect.
To reduce assembly time and material cost, manufacturers have produced numerous designs in recent years featuring the structure of a single printed circuit board (PCB) with all components on it. However, PCBs, which typically comprise copper clad, fiberglass and resin, are poor heat dissipaters. Several companies have addressed this problem by applying a rigid plate and thermal interface materials attached to the body of power components. Unfortunately, heat transfer is not significantly improved by this method due to the high thermal resistance of the packaging material of the power components.
Some suppliers offer direct metal contact to cooling media for lowering θja. However, each design is unique and costly.
One prior art example uses two mechanical clips to hold two pairs of ferrite cores onto the PCB, and a pair of plastic terminal standoffs to hold tight a combination of a single-board sub-assembly, thermal interface materials, and a base plate. The drain leads/terminals of power Metal-Oxide Semiconductor Field-Effect Transistors (MOSFET) faces the base plate instead of the PCB. The thermal mass of the base plate, as well as its wide contact area, results in a low θja. However, the mechanism that holds all parts together is the press fitting between the metal studs on the base plate and the two ribbed holes of each standoff. The fitting pressure reduces because either the plastic standoff is aged, or incorrect assembly process widens one of the ribbed holes, creating reliability issues.
A second prior art example has all electronic components and terminal pins mounted to a single PCB, with an option of adding four metal, thread inserts and a separate base plate. Each insert is installed through a non-plated, pre-drilled hole on the PCB. The threads allow screws from the bottom up to mount this assembly onto a customer's PCB. Alternatively, the threads allow screws from the top down to mount a heat sink onto the base plate. In this design, the vertical position of the base plate is well maintained, and the clearances between the base plate and each power-generating component are minimized. However, the disadvantage of this design is that part of the PCB must be reserved for the pre-drilled holes and inserts, thus reducing the available area for circuit layout. This is especially disadvantageous when the package size shrinks to industry standard 8th and even 16th bricks.
Another example comprises a single-board power module with flat heat spreaders and thermal interface materials. The thermally conductive, electrically insulated materials are filled among a spreader and power components. These power components may be soldered on the spreader. The thermal materials may also be treated as heat slugs, a kind of underfill. There may also be two or more spreaders on each side of the PCB, and a heat sink can be added right outside either spreader.
Yet another prior art example uses an add-on base plate mounted on top of the bare power module. The base plate is coated with a layer of electrical insulation and has four standoffs in the corners. In each standoff there is a half-way tapped hole matching another tapped hole on the PCB. A thermal interface material is filled between the base plate and power MOSFETs. In this example, the base plate is fastened to the module, and an additional heat sink with a variable fin height can be mounted onto the base plate. The disadvantage of this design is the complicated structure that results in extra labor and material costs, including power coating, different screw sizes, etc.
The final prior art example comprises a spring clip, a heat sink and a base housing with pin sockets. Although the clip provides a retaining force, cooling is not facilitated by the clip. The heat sink serves as the sole source of heat dissipation within the prior art. The clip in the prior art is designed such that air may flow freely through the clip thereby enhancing the ability the heat sink to dissipate heat from the underlying component or components, but the clip itself does not contact the chip heat source directly as a heat dissipating component.